This is an experimental research program to compare the respiratory tract deposition patterns of hygroscopic aerosols with the deposition patterns of stable aerosols in dogs and human subjects. The changes in air temperature, humidity, and hygroscopic aerosol particle size will be measured in the upper respiratory tract of the dog. Deposition patterns of radiolabelled stable and hygroscopic aerosols will be determined by an external imaging device. The influence of particle size, upper respiratory tract conditioning, and particle composition will be investigated in aerosol deposition experiments with dogs, normal humans, and respiratory disease patients.